1. Field of the Invention
The present invention relates to a drive method for a solid-state imaging device, a solid-state imaging device, and an imaging apparatus. More particularly, the invention relates to a drive method for an X-Y address solid-state imaging device, a typical example of which is a complementary metal-oxide semiconductor (CMOS) device image sensor, a solid-state imaging device implementing the above drive method, and an imaging apparatus using the solid-state imaging device.
The invention also pertains to a solid-state imaging apparatus and an imaging apparatus, and more particularly, to a solid-state imaging apparatus in which a color filter having a primary color component for generating luminance (Y) components and other color components is disposed on the surface of the pixels, and also to an imaging apparatus using the solid-state imaging apparatus as the imaging device.
2. Description of the Related Art
To improve the frame rate in a solid-state imaging device, generally, the amount of pixel information is decreased by adding information concerning a plurality of pixels, as disclosed in, for example, Japanese Unexamined Patent Application Publication No. 2004-266369.
An example of the above-described technique is as follows. In color coding of a Bayer pattern shown in FIG. 1, from a 3×3 pixel area, the same color of pixels in the two columns and the two rows are extracted and added while shifting the 3×3 pixel area by three pixels by maintaining the original pixel pattern without changing the color spatial repeat pattern or changing the pixel pitch ratio in the vertical, horizontal, and oblique directions.
Red (R) pixels 311, 313, 331, and 333 located in the odd-numbered rows are added, and then, the resulting addition R signal is positioned at centroid A. Similarly, by horizontally shifting three pixels from the R pixels 311, 313, 331, and 333, green (G) pixels 314, 316, 334, and 336 are added, and then, the resulting addition G signal is positioned at centroid B. By further horizontally shifting three pixels from the G pixels 314, 316, 334, and 336, R signals 317, 319, 337, and 339 are added, and then, the resulting addition R signal is positioned at centroid C.
Then, by vertically shifting three pixels from the R pixels 311, 313, 331, and 333, G pixels 341, 343, 361, and 363 located in the even-numbered rows are added, and then, the resulting addition G signal is positioned at centroid D. By horizontally shifting three pixels from the G pixels 341, 343, 361, and 363, blue (B) pixels 344, 346, 364, and 366 are added, and then, the resulting addition B signal is positioned at centroid E. In this manner, by adding color pixels as described above over the entire pixel area, the same colors of pixels can be added while maintaining the original color pattern without changing the color spatial repeat pattern or changing the pixel pitch ratio in the vertical, horizontal, and oblique directions.
In imaging apparatuses, such as in digital still cameras and video cameras, the number of pixels of solid-state imaging apparatuses used as imaging devices is increasing, and solid-state imaging apparatuses having several millions of pixels are coming into widespread use. The use of multi-pixel imaging devices aims to obtain high-resolution images. However, there is still a demand for solid-state imaging apparatuses exhibiting higher resolution.
In single-panel digital cameras, the color pattern of a color filter used in a solid-state imaging apparatus is very important to obtain high resolution. A typical example of the color pattern is the known, widely used Bayer pattern.
Bayer Pattern
The Bayer pattern is a color pattern, as shown in FIG. 2, in which a GR line having G pixels and R pixels alternately and a GB line having G pixels and B pixels alternately are disposed alternately in the horizontal direction (also in the vertical direction). The feature of this Bayer pattern is that the pixels are disposed in a square lattice at regular intervals d (pixel pitches) of the pixels in the vertical and horizontal directions and that the ratio G:R:B of the GRB colors in this square lattice pattern is 2:1:1.
The spatial frequency characteristics of the RGB colors in the Bayer pattern are now described by separately considering the characteristics of the G color, which is the primary color for generating luminance (Y) components, and the other colors, i.e., the R and B colors.
Generally, the luminance signal Y is generated according to equation (1).Y=0.6G+0.3R+0.1B.  (1)
Equation (1) is based on the fact that the human eye is more sensitive to the G color and less sensitive to the R and B colors. That is, if high resolution is necessary for the luminance (Y) components, it is very important to increase the resolution of the G color components, and not very high resolution is necessary for the other R and B color components.
FIGS. 3A and 3B illustrate the G pattern from which only G pixels are extracted from the Bayer pattern. The spatial frequency characteristics are now considered with reference to FIGS. 3A and 3B. If the pixel sampling rate is set to be the pixel pitch d, the sampling rate for the G pixels is equal to the pixel pitch d in the vertical and horizontal directions, and according to the sampling theorem, signal components having frequencies up to (1/2)fs (fs (=1/d): sampling frequency) can be collected. That is, it is possible to collect signal components indicated by the half-tone columns and the voided columns shown in FIG. 3A according to the theoretical threshold and it is not possible to collect signal components having higher frequencies beyond this threshold frequency.
Concerning the 45° oblique direction, since the sampling rate for the G pixels is d/√2, signal components up to (1/2√2)fs can be collected according to the sampling theorem.
Similarly, the spatial frequency characteristics of the R and B pixels are considered below. In this case, since the pixel pitches for the R and B pixels are the same, only the spatial frequency characteristics of the R pixels are described below.
The R pattern from which only the R pixels are extracted from the Bayer pattern is shown in FIGS. 3C and 3D. Concerning the spatial frequency characteristics of the R pixels, since the sampling rate for the R pixels is 2d in the vertical and horizontal directions, signal components having frequencies up to 1/4fs can be collected according to the sampling theorem. In the oblique direction, the sampling rate for the R pixels is d/√2, and thus, signal components having frequencies up to (1/2√2)fs can be collected according to the sampling theorem.
In FIGS. 3A through 3D, threshold frequency components that can be collected in the vertical, horizontal, and oblique directions are indicated by the voided columns and half-tone columns.
The spatial frequency characteristics of the G, R, and B pixels are shown in FIG. 4. FIG. 4 shows that, when the sampling rate is set to be the pixel pitch d (=1/fs), the spatial frequency characteristics of the G pixels exhibit the resolution up to 1/2fs in the vertical and horizontal directions and up to (1/2√2)fs in the oblique 45° direction and the spatial frequency characteristics of the R pixels exhibit the resolution up to 1/4fs in the vertical and horizontal directions and up to (1/2√2)fs in the oblique 45°direction, i.e., signal components up to the above-described threshold frequency can be collected.
Bayer Pixel Shifted Pattern
In addition to the above-described Bayer pattern, the pattern shifted by 45° from the Bayer pattern shown in FIGS. 3A through 3D, such as the pattern shown in FIGS. 6A through 6D, that is, a modified Bayer pattern in which pixels are disposed by being shifted by half the pixel pitch in the vertical and horizontal directions, has been proposed, as disclosed in Japanese Unexamined Patent Application Publication No. 10-262260.
The color pattern generated by shifting the Bayer pattern by 45° is hereinafter referred to as the “Bayer pixel shifted pattern”. In this Bayer pixel shifted pattern, since the sampling rate results in d/√2, which is 1/√2 times as high as the sampling rate d of the Bayer pattern, higher resolution can be obtained compared to that of the Bayer pattern.
From another point of view, if the same resolution is required in the Bayer pattern and in the Bayer pixel shifted pattern, the sampling rate of the Bayer pixel shifted pattern can be increased by √2 as large as that of the Bayer pattern. In other words, by using the Bayer pixel shifted pattern, the same resolution can be obtained with a smaller number of pixels than that in the Bayer pattern. As a result, the pixel aperture can be increased so that the photo-sensitivity of the pixels can be enhanced, thereby obtaining signals having a high signal-to-noise (S/N) ratio.
However, the Bayer pixel shifted pattern can exhibit high resolution only for achromatic subjects. The reason for this is as follows.
FIG. 5 illustrates color coding of the Bayer pixel shifted pattern.
The G pattern from which only the G pixels are extracted from the Bayer pixel shifted pattern is shown FIGS. 6A and 6B. Since the sampling rate for the G pixels in the vertical and horizontal directions is √2d, which is larger than the sampling rate d for the G pixels in the vertical and horizontal directions in the Bayer pattern, the resolution in the Bayer pixel shifted pattern is lower than that in the Bayer pattern. On the other hand, since the sampling rate d for the G pixels in the 45° oblique direction is smaller than the sampling rate d/√2 in the 45°oblique direction in the Bayer pattern, the resolution is higher than that in the Bayer pattern.
Similarly, the resolution of the R pixels and the B pixels is considered. Since the pixel pitches for the R pixels and the B pixels are the same, only the resolution of the R pixels is described below.
The R pattern from which only the R pixels are extracted from the Bayer pixel shifted pattern is shown in FIGS. 6C and 6D. The sampling rate for the R pixels in the vertical and horizontal directions is √2d, and the sampling rate for the R pixels in the oblique direction is 2d.
In FIGS. 6A through 6D, threshold frequency components that can be collected in the vertical, horizontal, and oblique directions are indicated by the voided columns and half-tone columns.
The spatial frequency characteristics of the G, R, and B pixels are shown in FIG. 7. Upon comparing FIG. 7 with FIG. 4, it is seen that the spatial frequency characteristics of the Bayer pixel shifted pattern are the same as those shifted from the spatial frequency characteristics of the Bayer pattern by 45°.